YouTube's new What to Watch page is a mess. It offers a mixture of updates from channels that you are subscribed to and popular channels on YouTube that the site's algorithm thinks you will like as well.
My main issue here is that there is apparently no way to block certain channels from being listed here.
While you can click on the three dots on the right side of the channel row -- it only appears when you hover the mouse cursor over the channel -- and select hide these videos from the menu that is displayed, it won't really help as the video suggestions appear again on the next reload of the page.
This is without doubt one of the most annoying features of YouTube. Why have an option to hide video suggestions if they are displayed again on reload?
I spend a lot of time this morning trying to figure out a way to resolve the issue. I tried several userscripts that promised to remove channels from YouTube but all did not work anymore.
Eventually though I found a solution that appears to work fine, so here it is:
Open the YouTube homepage. The What to Watch page should be displayed here by default. Locate a channel that you want to remove permanently.
Click on the channel name. This opens the profile page of that channel. Click on the About link here at the top of the page.
Here you find information about the channel. The interesting bit here when it comes to removing channels from YouTube's What to Watch page is the little flag icon in the lower left corner of the screen.
Click on the icon and select block users from the options menu.
Note that it takes a couple of reloads before the suggestion disappears. While I was not able to verify the following, it seemed to help to use the hide these videos option on the page to get rid of the channel suggestion completely.
While that removes the suggested video channels for good from YouTube's homepage, it won't remove the suggestions. This means that YouTube will display other channel suggestions to you instead which you may or may not like as well.
It is not possible to hide those suggestions on the page. The best way to resolve this is to always open the My Subscriptions page directly when you visit YouTube instead of the What to Watch page.